The use of an information carrier plate (also referred to as a phosphor plate or phosphor storage plate) as a detector for obtaining visually perceptible contrast upon exposure to X-rays is known in the art as computed radiography (CR) and is described for example in U.S. Pat. No. 7,211,785 entitled “SCANNING APPARATUS” to Berger et al., incorporated herein by reference. In Computed Radiography (CR), a phosphor carrier plate is exposed to x-ray or other short-wavelength ionizing radiation and stores a latent image that is read out by a scanning device.
The imaging cycle employing such plates as x-ray detectors comprises juxtaposing the phosphor plate nearby a specific part of the body (e.g., leg, arm, tooth, and the like) and then exposing the plate to X-rays in order to obtain an image from stored radiation energy. Following exposure, the phosphor plate is then removed from the patient and the latent image that is stored thereon is scanned using a stimulating laser beam or other energy source. When it receives the stimulating beam, the illuminated spot on the phosphor plate emits radiation at a second, shorter wavelength, typically in the blue region. The amount of radiation that is emitted upon stimulation is proportional to the amount of energy stored as a result of x-ray exposure. After the plate has been scanned, the obtained image data can be displayed and stored for further examination. The exposed and scanned plate is then erased and can be reused in a subsequent imaging cycle.
Among factors that determine the usefulness and quality of a radiographic image are proper placement of the detector relative to the object that is to be imaged and appropriate positional arrangement of the x-ray source, object, and detector. In conventional radiography, the object is placed between the x-ray radiation source and the detector such as the phosphor plate, and the relative positions of the source and detector are coordinated for proper alignment and angle for obtaining an image. When the object is an arm, leg, or chest of a patient, the x-ray tube, the object to be imaged, and at least portions of the detector are visible to the x-ray technician, so that the task of alignment is straightforward.
Alignment is difficult for dental or intraoral radiography. The detector position is within the patient's mouth and is ordinarily not visible to the technician. The technician typically places the detector into some type of holder, and then inserts the holder into place in the mouth. The holder may have a bite plate or other type of supporting member that helps to position the detector appropriately within the mouth. Holders of this type can be cumbersome and uncomfortable to the patient. Holders and other positioning devices are not error-proof, and positioning errors with these devices can mean that the images obtained are not suitable for use in detecting some types of problems. Poorly aligned detectors can be the cause of problems such as cone cuts, missed apices, and elongation and related angulation or parallax errors, for example. These alignment problems can require re-takes, additional image captures to acquire an acceptable image. Re-takes are undesirable due to the additional x-ray radiation exposure to the patient and because of prolonged patient discomfort with the detector held in the mouth for a longer time period.
Some x-ray sources have included aim indicators that help the technician adjust the position and angle of the x-ray source. Typically, these aim indicators use visible light to trace an outline that helps to center the radiation beam. These work where the radiation detector can be seen, but fall short of what is needed where the detector is not visible, such as with intraoral imaging. The technician must guess or estimate both the position of the intraoral sensor and the angle of incidence of x-rays on the sensor.
The simplified schematics of FIGS. 1A-1E show how mis-alignment between an x-ray source 10 and a detector 20 can occur. The object being imaged is not shown, since it is removed for improved clarity in describing the alignment problem. For reference in these examples, x-ray source 10 provides visible light aim indices 12 used for aim centering. When correct aim alignment is achieved, shown in FIG. 1A, detector 20 is centered, as shown within aim indices 12. Aim is incorrect at examples shown in FIGS. 1B and 1D.
Proper alignment with respect to angle, or angulation, is desirable. For many types of images, incident radiation from x-ray source 10 is preferably orthogonal to detector 20 as shown in the FIG. 1A example. Line N in FIG. 1A indicates a normal, or orthogonal line, to the surface of detector 20. Examples in FIGS. 1C and 1D show incorrect angular alignment. In example FIG. 1C, aim is correct but angulation is incorrect. In the example of FIG. 1D, both aim and angulation are incorrect. In the example of FIG. 1E, detector 20 is rotated in its own plane.
Note that the schematic examples of FIGS. 1A and 1B assume an orthogonal positioning of x-ray source 10 to detector 20. In some embodiments, an oblique orientation may be preferable. This can complicate the alignment task, since it can be difficult to obtain the desired oblique angle for a detector 20 that is not visible when the patient's mouth is closed.
Positioning a sensor relative to the x-ray source is described in U.S. Pat. No. 7,780,350 entitled “POSITIONING ADJUSTMENT OF A MOBILE RADIOLOGY FACILITY” to Tranchant et al.
At least one drawback of alignment methods relates to lack of guidance for correcting for mis-alignment. The technician needs information in order to correct for mis-alignment and to verify that proper alignment has been obtained. Some methods for reporting the alignment information, such as providing information on an operator console, for example, can be difficult to use when making position adjustments. The technician needs to move back and forth between the operator console and the x-ray tube, checking and correcting each adjustment until proper alignment is achieved.
Thus, there is a need for an apparatus and method for providing improved alignment of the radiation source and image detector in intraoral radiography.